The Boston Marathon has been hit by two explosions on the finish line, leaving at least three people dead and dozens more injured.

The incident is thought to have occurred at about 2.50pm Boston time, around three hours after the winner had crossed the finish line.

Many of those injured were spectators, with reports suggesting that many of the victims were missing limbs.

"We started grabbing tourniquets and started tying legs. A lot of people amputated... At least 25-30 people have at least one leg missing, or an ankle missing, or two legs missing," revealed one runner.

Both explosions came on Boylston Street near the finish line of the race, the last mile of which had been dedicated to the Newtown massacre.

"There was an explosion, police, fire and EMS are on the scene. We have no indication of how many people are injured," said a spokeswoman for the Boston Police Department.

However, the police have since confirmed that at least three people have lost their lives, with some sources claiming that up to 100 people have been wounded, many critically.

Two more explosives have also reportedly been found on the route and are being dismantled, while a fire at JFK library is thought to be unrelated.

A disaster response centre has been set up in the area, while many of the victims have been taken to Massachusetts General Hospital.

New York police have stepped up security around landmarks and prominent hotels in response to the blasts.

The Boston Marathon is the oldest annual marathon in the world and around 25,000 people had registered to run at this year's event.